Segment Information (Details) $ in Millions	3 Months Ended		5 Months Ended	9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 segment	Sep. 30, 2017 USD ($) segment revenue_line	Sep. 30, 2016 USD ($)
Segment Reporting Information
Number of reportable segments | segment				1
Number of revenue lines | revenue_line				5
Number of operating segments | segment			1	1
Total revenue	$ 2,340	$ 2,201		$ 7,089	$ 6,759
Intersegment Eliminations
Segment Reporting Information
Total revenue	(5)	(3)		(9)	(6)
Commercial Risk Solutions | Operating Segments
Segment Reporting Information
Total revenue	917	884		2,943	2,835
Reinsurance Solutions | Operating Segments
Segment Reporting Information
Total revenue	355	329		1,070	1,032
Retirement Solutions | Operating Segments
Segment Reporting Information
Total revenue	491	466		1,266	1,266
Health Solutions | Operating Segments
Segment Reporting Information
Total revenue	293	265		977	838
Data & Analytic Services | Operating Segments
Segment Reporting Information
Total revenue	$ 289	$ 260		$ 842	$ 794